The present invention relates to a method of anti-skid brake control for controlling skid of wheels by repeatedly decreasing, holding and increasing braking pressure during braking operation.
In general, anti-skid brake control is performed, when it is detected that wheels tend to be locked during braking, to eliminate the locking tendency by decreasing braking force of the wheels and by increasing the braking force again thereafter in order to stabilize driving of the vehicle and to make the braking distance as short as possible.
One of conventional type general anti-skid brake control systems is explained by FIG. 3.
As shown in FIG. 3, a brake control circuit, for which the above anti-skid brake control system is applied, comprises a brake valve 1 operated by a driver, an air tank 2 where compressed air is accummulated by an air compressor (not shown), a relay valve 3 for supplying the compressed air of the air tank 2 to a brake chamber when brake signal air pressure is sent from the brake valve 1, a holding valve 4 consisting of a permanently open solenoid valve mounted in an air passage between the brake valve 1 and the relay valve 3, an exhaust valve 5 consisting of a permanently closed solenoid valve mounted in an air passage between the holding valve 4 and the relay valve 3, a check valve 6 mounted in an air passage between the brake valve 1 and the relay valve 3 by bypassing the holding valve 4 and allowing only the air to pass from the relay valve 3 to the brake valve 1, and a controller 7 for controlling opening and closing of the holding valve 4 and the exhaust valve 5.
When the driver opens the brake valve 1 to brake when the vehicle is driven, brake signal air pressure by compressed air of the air tank 2 is sent into a pilot chamber of the relay valve 3, and the relay valve 3 is switched to a position to communicate the air tank 2 with the brake chamber. Then, the compressed air of the air tank 2 is introduced into the brake chamber, and brake is operated. In this case, braking pressure gradually increases as shown in FIG. 4. During braking, a controller 7 computes wheel deceleration based on wheel speed as supplied and, by detecting that the computed wheel deceleration reaches a threshold value .alpha., judges that the wheels are being locked. Then, the holding valve 4 is turned on, and it is closed. As the result, the supply of the brake signal air pressure to the relay valve 3 is stopped, and the relay valve 3 stands at neutral position, and the brake chamber is shut off from the air tank 2 and exhaust outlet. As the result, braking pressure is maintained at approximately constant level.
Even when this braking pressure is maintained, if the controller 7 detects that the wheel deceleration gradually increases and reaches the threshold value .beta., it judges that the wheels are locked. Thus, exhaust valve 5 is turned on and is opened. As the result, the compressed air, i.e. the brake signal air pressure supplied to the relay valve 3, is discharged, and the relay valve 3 communicates the brake chamber with the exhaust outlet. As the result, the compressed air in the brake chamber is discharged, and braking pressure is reduced.
When the braking pressure is reduced, wheel deceleration is decreased. The controller 7 judges that the wheels tend to be locked when it is detected that wheel deceleration reaches the threshold value .gamma.. Thus, the exhaust valve 5 is turned off and is closed. Then, the discharge of the compressed air supplied to the pilot chamber of the relay valve 3 is stopped. The relay valve 3 is at neutral position, and the brake chamber is shut off from the air tank 2 and the exhaust outlet. As the result, the braking pressure is maintained at approximately constant level.
After the braking pressure is turned to the holding state, if it is detected that the wheel deceleration is decreased and reaches the threshold value .delta., the controller 7 judges that the locking tendency has been eliminated. Thus, the holding valve 4 is turned off and is opened. Then, the compressed air is supplied to the relay valve 3 as brake signal air pressure, and the relay valve 3 communicates the air tank 2 with the brake chamber. As the result, the compressed air is introduced into the brake chamber, and the braking pressure is again increased.
As described above, anti-skid brake control is performed by repeatedly increasing, holding, decreasing, holding and increasing the braking pressure.
However, when braking pressure is controlled from pressure increase to pressure holding, braking pressure is overshot due to operation characteristics such as responsiveness of the relay valve 3. Braking pressure is not maintained in the preset holding pressure during pressure holding as shown by broken line in FIG. 1, and it is held at higher pressure than the preset holding pressure. For this reason, the wheels are now more easily locked. This overshooting is particularly high during the first control from pressure increase to pressure holding after starting braking operation, and it is relatively low for the second time and after because it is already under anti-skid brake control.
During the control from pressure decrease to pressure holding, braking pressure is often undershot. Under pressure holding condition as shown by broken line in FIG. 2, braking pressure is not held at the preset holding pressure and it is held at lower pressure than the preset holding pressure. Thus, the restoration of wheel deceleration is delayed, and the braking distance becomes longer.
As described above, in the conventional anti-skid brake control, controllability is not always satisfactory due to operating characteristics of the relay valve.